Cross-reference is made to a related U.S. patent application by Thomas L. Tapp and Joseph T. Graf, filed on even date herewith, and entitled xe2x80x9cBlock Decision Feedback Equalization Method and Apparatus.xe2x80x9d
The present invention generally relates to an improved data communication system and technique, more particularly relates to an improved equalization scheme for a digital communication system, and even more particularly relates to block decision directed equalization of signals transmitted via a dynamic dispersive media.
Equalization is a technique often used in wireless communication applications. For example, when digital data is transmitted over a dispersive radio channel, an equalizer can be used to improve the estimate of the originally transmitted symbols. More particularly, equalizers are becoming increasingly important in high frequency (HF) communication applications. Such applications are requiring increasingly higher data rates in a fixed bandwidth; i.e. increased bandwidth efficiency. For example, many current applications require performance sufficient to meet the specifications for a 64-QAM (quadrature-amplitude modulation) signal.
As bandwidth efficiency is increased, however, information reliability is typically reduced. The reduction in reliability results, at least in part, from various channel effects. Equalization techniques can be used to mitigate many of the various channel effects. Use of an equalization technique can thereby make the communication system more reliable.
Several different channel effects can cause a reduction in reliability. A description of some of the various channel effects follows. Constrained channel bandwidth, for example, can cause time dispersion and intersymbol interference (ISI). Thermal noise and interference can lower the signal-to-noise ratio and thereby impact performance. Multipath interference can cause fading and time dispersion. Fading can also result from the physical characteristics of the communication channel being used. Delay and Doppler shift, which also result from the physical characteristics of the communication channel being used, can cause, respectively, time and frequency shifts.
Transmitter and receiver characteristics, which also affect the signal, must be accounted for as well. For example, the transmitter can include transmit filters and automatic level control (ALC). Likewise, the receiver can include receive filters and automatic gain control (AGC). These system influences should be addressed since they are also included in the channel response of any real system.
The equalizer""s role is to compensate for such effects. FIG. 1 depicts a block diagram of a basic communication system. Data from a source 100 is fed to a modulator 102 that in turn passes the modulated data to a wireless communication component 104. The communication signal is then transmitted by a transmitter antenna 106 to a receiver antenna 108. The link between the two antennas 106, 108 is known as the propagation channel 110. The receiver antenna 108 passes the received signal to the receiver""s wireless communication component 112 that in turn sends the signal to a receiver-based demodulator 114. The demodulated signal is then sent for further processing, handling or storage as represented by sink 116.
The channel between the two wireless communication components 104, 112 is known as the radio channel 118. The channel between the modulator 102 and the demodulator 114 is known as the modulation channel 120. The channel between the source 100 and the sink 116 is known as the digital channel 122. The equalizer typically compensates for the influences found across the modulation channel 120. Thus, when used for such a purpose, it can be used to compensate for the channel and system effects described above.
FIG. 2 depicts a use of an equalizer as a filter compensating for the effects influencing the modulation channel. The effects of the transmitter 200 can be represented as HTx(f). The effects of the propagation channel 202 can be represented as C(f). The effects of the receiver 204 can be represented as HRx(f). For example, the ideal response of a linear equalizer 206 can be represented as a filter that mitigates the effects of the modulation channel by having a response such as [HTx(f) C(f) HRx(f)]xe2x88x921.
Typical equalizer implementations require knowledge of the characteristics of the communication channel. Several different techniques have been used to estimate or model these characteristics. For example, the initial channel characteristics can be estimated using a training sequence. The estimate can then be updated using a least-mean squares (LMS) algorithm or a recursive-least squares (RLS) algorithm. Another technique employs periodically inserted correlator probes, which are used to guide the channel updates. Further, use of a carefully designed probe can enable an efficient estimation of the channel characteristics. One example of such a probe is provided in the MIL-STD-188-110B Appendix C standard, wherein a simple correlator is used to obtain the channel estimate.
An example of a typical signaling sequence is depicted in FIG. 3. An initial preamble 300 of xe2x80x9cxxe2x80x9d symbols is used for acquisition and synchronization. In this example, the data packets 302, 304 each include thirty-two unknown symbols. The probes 306, 308 each include a sequence of sixteen known symbols. Alternatively, different numbers of symbols can be communicated in the unknown data segments 302, 304 and probes 306, 308. For example, in the more recently developed MIL-STD-188-110B Appendix C standard, each known probe segment includes thirty-one known symbols and each unknown data segment 302, 304 includes 256 unknown symbols. Each xe2x80x9cunknownxe2x80x9d data segment 302, 304 is thus encapsulated between two xe2x80x9cknownxe2x80x9d probe packets 300, 306, 308. Various modulation techniques, such as phase-shift keying (PSK), quadrature amplitude modulation (QAM) and quadrature phase-shift keying (QPSK), for example, can be used to transmit the various segments of the signaling sequence.
An example of a communication system using an encapsulated unknown data/known probe approach, such as that depicted in FIG. 3, can be found in U.S. Pat. No. 4,365,338 (Technique For High Rate Digital Transmission Over A Dynamic Dispersive Channel) to McRae et al. In the technique disclosed in U.S. Pat. No. 4,365,338, the channel characteristics are assumed to be fixed over the extent of a given unknown data segment. This approach, however, often does not reflect the true state of the system. In reality, the channel characteristics can vary over time throughout the duration of the data segment, especially as the length of the unknown data segment is increased.
Consequently, there exists a need for an equalization approach that more closely models the real world behavior of the channel characteristics. More particularly, an approach is needed that more fully accounts for the channel characteristic variance present throughout the duration of a data segment. These needs and others are fulfilled by the invention disclosed in the following detailed description.
It is an object of the present invention to provide an improved equalizer for a communication system transmitting data via a dispersive media.
It is a feature of the present invention to utilize a block equalizer capable of compensating for channel characteristics that vary over the duration of an unknown data segment.
It is an advantage of the present invention to provide a more accurate and realistic estimation of the channel characteristics present over an unknown data segment, thereby increasing reliability in determining the unknown symbols.
Thus, the present invention involves an improved block equalization apparatus and method. It is carried out in a non-restrictive manner in the sense that there are separate solutions that do not directly influence each other""s decisions. The channel estimate is not assumed to be constant. The invention can handle environments wherein the channel varies significantly over the duration of the unknown data segment. Further, this approach provides increased flexibility since the estimates can be combined in a variety of different ways.